Method for Monitoring Fouling in a Cooling Tower

ABSTRACT

Fouling in the fill portion of a cooling tower is monitored by transmitting radiation through a cooling tower, detecting the amount of radiation that has penetrated the cooling tower, and calculating the density of the fill portion of the cooling tower based on the detected radiation. A higher than expected density indicates the presence of fouling on the fill portion of the cooling tower. A rate of fouling may be established by monitoring the density of the fill portion of the cooling tower over time.

This application claims priority from U.S. Provisional Application Ser.No. 60/979,081, filed on Oct. 11, 2007, which is hereby incorporated.herein by reference.

BACKGROUND

The present invention relates to cooling towers. More particularly, thepresent invention relates to direct or open-type cooling towers. Coolingtowers rely on evaporation to remove heat from a stream of water (orother medium). In open cooling towers, the water to be cooled is exposeddirectly to the atmosphere. Typically, the warm water is sprayed overthe top of a “fill” portion in the cooling tower while ambient air isblown through the fill. The fill is used to increase the contact areabetween the warm water and the (cooling) air, thereby providing greaterheat transfer.

One problem associated with cooling towers is the build up of scaledeposits on the fill (i.e. fouling). Minerals dissolved in the coolingwater accumulate on the fill as the water evaporates. Buildup or foulingcan significantly reduce the heat transfer and, therefore, reduce theefficiency of the cooling tower. Further, excessive fouling can evencause the fill portion to collapse due to the additional weight of thefouling material. It is therefore desirable to monitor the amount offouling or buildup that occurs on the fill.

SUMMARY

In one embodiment of the present invention, a radioactive source-and adetector are placed on opposite sides of a cooling tower. The detectormeasures the amount of radiation transmitted through the fill in thecooling tower. The transmitted radiation is then converted to a densityvalue. The measured density is then compared to a baseline density of(just) the fill in order to determine if fouling is present (i.e. adensity that is higher than expected would indicate the presence ofbuild-up or scaling). Further, the density may be measured at periodicintervals to determine a rate of fouling.

The technique is extremely non-intrusive because the measurements aretypically taken along the external surfaces of the cooling tower whilethe cooling tower is in operation. Thus, in-situ density values may beobtained. Further, internal access to the cooling tower is notnecessary, so there is minimal disruption to the process system. Also,the measurements may be taken at many different points through the fillmaterial, yielding an overall “profile” of the cooling tower. As such,the technique does not rely on one measurement or sample point beingrepresentative of the entire fill, and asymmetrical or non-uniformscaling patterns can be readily detected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side schematic view of a cooling tower showing the equipmentused to perform the method of the present invention; and

FIG. 2 is a plot of radiation intensities detected for a typical coolingtower

DETAILED DESCRIPTION

In one embodiment of the present invention, a method of monitoringfouling in a cooling tower is described. FIG. 1 shows a typical coolingtower 10. In this case, the cooling tower 10 is a cross-flow design withthe flow of ambient air represented by the wide arrows 12 and the flowof water represented by the narrow arrows 14. Of course, the method alsomay be used with other cooling tower designs, such as counter-currentflow types.

With reference to FIG. 1, a supply of hot water enters the cooling tower10 through water inlets 16. The water is distributed over the fill 18 bydistribution plates 20. Other means for distributing the water, such asa spray header, are also common. The water falls down through the fill18, where it is cooled through the flow of air. The cooled watercollects in a basin 22 below the fill 18 and exits the cooling tower 10through a water outlet 24. A fan 26 helps drive the flow of air throughthe fill 18.

To monitor the fouling that occurs in the fill 18, a radioactive source30 and a radiation detector 40 are positioned on opposite sides of thecooling tower 10. Preferably, the source 30 is a gamma-ray emittingsource, such as Cesium-137, Cobalt-60 or Sodium-24. The activity of thesource 30 is chosen based on the dimensions of the particular coolingtower 10. That is, the source 30 needs to be strong enough to be able topenetrate though the cooling tower 10 (and the fill 18) to the oppositeside (where the detector 40 is positioned). Typically, the activity ofthe source 30 is between 50 millicuries (mCi) and 500 millicuries (mCi).

The detector 40 detects the gamma rays transmitted through the coolingtower 10 from the gamma source 30. Of course, if another type ofradioactive source (i.e. x-ray emitting) is used, a suitable detector isused. A typical gamma ray detector is a 2-inch sodium iodidescintillation detector, such as the ones manufactured by Ludlum.

Although not shown in FIG. 1, a radiation counting device receives thesignal from the detector 40. A Model 2200 Scalar Ratemeter by Ludlum isa typical type of counting device. The detector 40 and counting devicemeasure the intensity of the gamma radiation transmitted through thecooling tower 10. For example, a typical counting device may display themeasurement of the radiation intensity as counts of radiation perspecified time period (e.g. 5,000 counts/6 seconds). A dashed line 50 isrepresentative of the path of the radiation along which the intensity ismeasured.

According to the basic principles of radiation, the intensity of theradiation decreases as it passes through an absorbing material (e.g. thefill in the cooling tower). The decrease is dependent on the density andthickness of the absorbing material according to the following formula:

I=I _(o) e ^(−ρμx)

-   -   where I_(o) is intensity of the initial radiation        -   I is the intensity after passing through the absorbing            material        -   ρ is the density of the absorbing material        -   x is the thickness of the absorbing material and        -   μ is an absorption coefficient

Thus, the measured radiation can be converted into density, and theresults may be reported as such. Abnormally high density measurementsindicate the presence of additional material (e.g. fouling). The densityreadings can also indicate the severity of fouling.

In a typical method for measuring the amount of fouling, the source 30and detector 40 are positioned near the top of the cooling tower 10. Thesource 30 and detector 40 are each suspended from cables, wires or thelike that are routed through a pulley system, which, in turn, allows thesource and detector to be simultaneously raised or lowered along thesides of the cooling tower. Of course, other means for simultaneouslyraising or lowering the source and detector could alternatively be used.The source 30 and detector 40 are aligned with each other, such that thepath of radiation (e.g. dashed line 50 in FIG. 1) is substantiallyhorizontal. Beginning at the top of the cooling tower 10, the source 30and detector 40 are simultaneously lowered past the fill 18 insuccessive increments. As the source 30 and detector 40 are lowered, theintensity of the radiation is noted or recorded. For instance, theradiation readings may be recorded by a computer. Generally, the sourceand detector are lowered in 3 inch increments. In other words,measurements are taken every 3 inches from top to bottom. Further, eachindividual reading is taken for a designated time interval. In mostcases, the time interval for the readings is between 3 seconds and 6seconds. FIG. 2 shows a typical representation of the radiationintensities measured through the cooling tower. The x-axis is theradiation intensity (e.g. counts of radiation per 6 seconds) and they-axis is the elevation (e.g. feet). From the information displayed onthe density profile of FIG. 2, the density value of the fill portion ofthe cooling tower can be calculated.

FIG. 1 shows the source 30 and detector 40 (with a dashed line 50representing the path of the radiation) at a first position above thefill portion 18 of the cooling tower 10. In a second position, a dashedline 50′ represents the path of radiation between the source 30′ anddetector 40′ through the fill portion 18 of the cooling tower 10.

The first and second positions shown are indicative of the relativelocations at which the radiation intensity may be measured in order tocalculate an in-situ density value for the fill 18. The in-situ densityrefers to the density of the fill under current conditions (i.e. thedensity of the fill currently inside the cooling tower at the time themeasurements are made). The first position is representative of ameasurement of the amount of radiation detected through a portion of thecooling tower that does not contain fill, and the second position isrepresentative of a measurement of the amount of radiation detectedthrough the fill portion of the cooling tower. An in-situ density of thefill may be calculated from these measurements and the equation providedearlier. That is, a measurement of the radiation intensity through thefill portion would be I (e.g. 200-500 counts/6 seconds) and themeasurement of the radiation intensity through the non-fill portionwould be I_(o) (e.g. 40,000 counts/6 seconds). The equation may berearranged to solve for the density of the fill portion, as shown below,where x is the thickness of the fill between the source and detector,and μ is an absorption coefficient

$\rho = \frac{{- \ln}\frac{I}{I_{o}}}{\mu \; x}$

Other than the presence of the fill, there are not many variablesaffecting the measurement taken at the first position compared to themeasurement taken at the second position. For instance, the distancebetween the source and detector is unchanged, and the external structureof the cooling tower is unchanged. As such, it improves the accuracy ofthe density determination. That is, the accuracy is improved over asituation where there is not a non-fill portion of the cooling tower orit is not feasible to measure a non-fill portion of the cooling tower.

Once an in-situ density value of the fill portion of the cooling toweris calculated, it is compared to an established baseline density valuefor the fill portion in order to determine if fouling is present. If thein-situ density is greater than the baseline density, then it tends toindicate that fouling is present. Through repeated density measurements,the degree of fouling may also be established. For instance, over time,it is anticipated that periodic in-situ density measurements canestablish a range of fouling, such as slight, moderate, severe orcritical. For example, an in-situ density value that is 2 lbs/ft³ higherthan the baseline density value may be classified as slight fouling, 4lbs/ft³ may be classified as moderate fouling, and so on.

The baseline density value may be established in a variety of ways. Forinstance, the baseline density value may be established throughappropriate specifications for the fill, in light of the operatingconditions. That is, one may look up the density of the fill fromspecifications provided by the manufacturer of the fill, while adjustingthe density to account for the rate at which water is circulatingthrough the cooling tower. In other instances, the baseline densityvalue may be established by measuring a sample of the fill. For example,a sample of a known volume of the fill may be weighed.

However, it is preferred that the baseline density value be establishedby transmitting and detecting radiation through the cooling tower whenit is expected to be free of fouling. For example, a cooling tower thathas just been thoroughly cleaned or a brand new cooling tower may beassumed to be free of fouling. Baseline density readings may beestablished by transmitting and detecting radiation through the new orrecently cleaned cooling tower (preferably, through the fill portion andthrough a non-fill portion) and calculating the density of clean fillusing the previously mentioned formula. A later in-situ density valuecan be measured and calculated in the same manner as the baselinedensity value to provide an accurate representation of the change indensity (if any).

Over time, a number of in-situ density values may be established for thefill portion of a particular cooling tower by periodically transmittingand detecting radiation through the cooling tower. Further, the densityvalues may be tracked over time to yield a rate of fouling in the fillportion. This information can be extremely useful in scheduling andplanning maintenance shut-downs.

In one method of the present invention, the effectiveness ofanti-fouling agents may be studied. It is known in the art that addingan anti-fouling agent to the water supply of a cooling tower can helpremove and/or control fouling that occurs through the fill portion.However, there is a wide array of anti-fouling agents available, and itis difficult to verify the effectiveness of a particular anti-foulingagent. In one method, the effectiveness can be established by measuringa first density value of the fill portion by transmitting and detectingradiation through the cooling tower, then adding a quantity of ananti-fouling agent to the water supply, then measuring a second densityvalue of the fill portion by transmitting and detecting radiationthrough the cooling tower after the anti-fouling agent has been added;and then comparing the second density value to the first density value.A decrease in density indicates that the anti-fouling agent is effective(because the fouling is reduced). A larger decrease in density wouldindicate a larger reduction of fouling.

The effective quantity of anti-fouling agent to be added can beoptimized. For instance, after a second density value is measured andcompared to the first density value, an additional quantity ofanti-fouling agent is added. Then a third density value is measured andcompared to the first and second density values. If, for instance, thereis no decrease in density between the second and third density values,then it may be concluded that the additional quantity of fouling agentwas unnecessary and wasteful. After repeating the steps of addinganti-fouling agent and then measuring a density value of the fillportion (by transmitting and detecting radiation through the coolingtower), a correlation between the quantity of anti-fouling agent and theresulting change in the density value of the fill portion may beestablished. With this correlation, the optimal amount of anti-foulingagent that should be added to the cooling tower water supply can bepredicted from a measured density value. As such, the anti-fouling agentmay be conserved.

Another benefit of the method of the present invention is that readingsmay be taken at several locations. For instance, once a first densityprofile from top to bottom is recorded, the source, detector, andaccompanying equipment may be moved to a new position and again loweredin successive increments to obtain a second scan profile. This techniquemay be repeated, as desired. As such, the technique does not rely on onemeasurement or sample point being representative of the entire fill, andasymmetrical or non-uniform fouling can be readily detected. Forinstance, FIG. 2 shows a profile of radiation intensities for a typicalcooling tower. The profile reveals that the intensity of radiation (i.e.the “counts”) through the fill portion is inconsistent from top tobottom. There is a noticeable gradient of increasing intensities(decreasing density values) from the top of the fill downward, with thelowest intensities (and highest density values) situated just below themiddle of the fill. By transmitting and detecting radiation at a numberof points, it is less likely that a problem area will be missed.

It will be obvious to those skilled in the art that modifications may bemade to the embodiments described above without departing from the scopeof the invention as claimed.

1. A method for monitoring fouling in the fill portion of a coolingtower comprising the steps of: establishing a baseline density value forthe fill portion of a cooling tower; transmitting radiation through thecooling tower; detecting radiation that has penetrated through thecooling tower; calculating an in-situ density value for the fill portionof the cooling tower based on the detected radiation; and comparing thein-situ density value to the baseline density value to determine adegree of fouling in the fill portion of the cooling tower.
 2. A methodfor monitoring fouling in the fill portion of a cooling tower as recitedin claim 1, wherein said baseline density value is established bytransmitting and detecting radiation through the cooling tower when itis expected to be free of fouling.
 3. A method for monitoring fouling inthe fill portion of a cooling tower as recited in claim 1, wherein saidbaseline density value is established from specifications for thedensity of the fill in the fill portion of the cooling tower.
 4. Amethod for monitoring fouling in the fill portion of a cooling tower asrecited in claim 1, wherein said in-situ density value is calculatedfrom the amount of radiation detected through the fill portion of thecooling tower and the amount of radiation detected though a portion ofthe cooling tower that does not contain fill.
 5. A method for monitoringfouling in the fill portion of a cooling tower as recited in claim 4,wherein said in-situ density value is calculated according to thefollowing equation:I=I _(o) e ^(−ρμx) where I_(o) is the amount of radiation detectedthrough a portion of the cooling tower that does not contain fill I isthe amount of radiation detected through the fill portion ρ is thecalculated density of the fill portion of the cooling tower x is thethickness of the fill portion; and μ is an absorption coefficient.
 6. Amethod for monitoring fouling in the fill portion of a cooling tower asrecited in claim 1, and further comprising the steps of: calculating aplurality of in-situ density values by transmitting and detectingradiation through the cooling tower over time; and determining a rate offouling of the fill portion of the cooling tower by tracking saidin-situ density values.
 7. A method for monitoring fouling in the fillportion of a cooling tower as recited in claim 1, wherein said radiationis gamma radiation.
 8. A method for monitoring fouling in the fillportion of a cooling tower, comprising the steps of: providing a coolingtower, said cooling-tower including a fill portion and a supply of watercontacting said fill portion; measuring a first density value of thefill portion by transmitting and detecting radiation through the coolingtower; adding a quantity of anti-fouling agent to the water supply;measuring a second density value of the fill portion by transmitting anddetecting radiation through the cooling tower after said anti-foulingagent has been added; and comparing the second density value to thefirst density value in order to determine the effectiveness of theanti-fouling agent.
 9. A method for monitoring fouling in the fillportion of a cooling tower as recited in claim 8, and further comprisingthe steps of: adding an additional quantity of anti-fouling agent to thewater supply; establishing a third density value of the fill portion bytransmitting and detecting radiation through the cooling tower aftersaid additional quantity of anti-fouling agent has been added; andcomparing the third density value to the first and second density valuesin order to determine the effectiveness of the additional quantity ofthe anti-fouling agent.
 10. A method for monitoring fouling in the fillportion of a cooling tower as recited in claim 9, and further comprisingthe steps of: repeating the steps of adding a quantity of anti-foulingagent and then establishing a density value of the fill portion bytransmitting and detecting radiation through the cooling tower; andestablishing a correlation between the quantity of anti-fouling agentadded to the water supply and the resulting change in the density valueof the fill portion.
 11. A method for monitoring fouling in a coolingtower comprising: providing a cooling tower, said cooling towerincluding a fill portion and a non-fill portion; placing a radioactivesource and a detector on opposite sides of said cooling tower, whereinsaid radioactive source and said detector are in horizontal alignment;establishing a first radiation intensity by transmitting and detectingradiation through the non-fill portion of the cooling tower;establishing a second radiation intensity by transmitting and detectingradiation through the fill portion of the cooling tower; calculating anin-situ density of the fill portion of the cooling tower using thefollowing equationI=I _(o) e ^(−μμx) where I_(o) is the first radiation intensity I is thesecond radiation intensity ρ is the in-situ density of the fill portionof the cooling tower x is the thickness of the fill portion; and μ is anabsorption coefficient; and comparing the calculated in-situ density ofthe fill portion to an expected density for a fill portion containingclean fill.
 12. A method for monitoring fouling in the fill portion of acooling tower as recited in claim 11, wherein said radioactive source isCesium-137.
 13. A method for monitoring fouling in the fill portion of acooling tower as recited in claim 11, wherein said radioactive source isCobalt-60.
 14. A method for monitoring fouling in the fill portion of acooling tower as recited in claim 11, wherein said radioactive source isSodium-24.
 15. A method for monitoring fouling in the fill portion of acooling tower as recited in claim 13, wherein said radioactive sourcehas an activity between 50 millicuries and 500 millicuries.